The leased circuit and public phone service have played an important role for as long as 100 years or more in a mission-critical task of an enterprise and in a field such as national defense in which the mission-critical communication is required.
On the other hand, with the recent rapid increase of communication traffic, a packet-switched network which facilitates the high-speed communication has explosively spread, and the price of the parts thereof has been rapidly decreasing by mass production. Furthermore, in the packet-switched network, by using VPN technology such as MPLS (Multi-Protocol Label Switching), users can be accommodated in the network in which a plurality of users are completely separated logically, and both the cost reduction and the reliable security by sharing of the device can be achieved. Therefore, it has been desired to replace the leased circuit and public phone service whose maintenance cost has been increased due to aging and whose communication speed is difficult to increase with the packet-switched network so as to increase the communication speed and the communication capacity. In response to this, international organizations for standardization such as IETF (Internet Engineering Task Force) and ITU-T (International Telecommunication Union-Telecommunication sector) have started the development of MPLS-TP (MPLS Transport-Profile) as a suitable method for accommodating the conventional leased circuit and public phone service. The MPLS-TP places the greatest emphasis on the reliability which is the biggest disadvantage of the packet-switched network, and the standardization has been developing mainly around OAM (Operation Administration and Maintenance) for fault detection and APS (Automatic Protection Switching) for communication path switching. In particular, OAM which plays a major role on the fault detection and the failure part determination is the technology to be the core of the reliability.
The protocol referred to as MPLS encapsulates the packet with one or plurality of headers called label, and transfers the packet at the sight of only the label at intermediate nodes (LSR: Label Switching Router) in the network. Therefore, the speeding up can be easily achieved in the architecture. In the case of the packet provided with a plurality of labels, LSR of the normal MPLS uses the LSP (Label Switched Path) ID included in the outermost (first) label as a key for transfer. On the other hand, the device (LER: Label Edge Router) to be an edge point of the path identifies the user and the service from the MPLS label added to the packet, delete the label, takes out the payload, and carries out the subsequent processes (transfer, termination and others). As described above, the MPLS is the protocol that connects the LERs to be the edge points of the network with the line (path) through a plurality of LSRs, and the path is designated by the first MPLS label for transfer. MPLS-TP places the greatest emphasis on maintaining compatibility with the MPLS, and the OAM added newly is also designed to comply with this architecture.
The function of OAM is roughly classified into, for example, fault detection, failure part determination, and performance measurement of the network. In particular, in view of the application to the leased circuit service in which high operation ratio is required, it is indispensable to minimize the communication down time due to serious fault that needs repair and replacement of device parts, and the OAM function that promptly determines the fault location plays an important role. The OAM that determines the fault location includes LB (Loop Back), LT (Link Trace), and PT (Path Trace) in which loopback is carried out at the intermediate node to test the connectivity. However, as described above, since the MPLS is the protocol that designates the path (line) in the network, it has a feature that it is difficult to designate the loopback location in the network with a point.
For this reason, in Ping (RFC 4379: Non-Patent Document 1) used in the conventional MPLS and the like, the loopback location is designated by using TTL (Time To Live) existing in the MPLS label for transfer. TTL is originally introduced in order to prevent the packet from infinitely staying in the network when a loop occurs in the network, and the packet is discarded when the packet goes through (hops) a finite counter. Each time when the packet hops LSR, TTL of the first MPLS label for transfer is subtracted by “1”, and when it becomes “0”, the corresponding packet is discarded in the device. By utilizing this feature, in the conventional Ping of MPLS and Trace Route, TTL of the first MPLS label is set so as to be “0” just at the node where the node which transmits the OAM packet becomes the loopback point, thereby achieving the designation of the loopback point. In LSR to be the loopback point, TTL of the first MPLS label for transfer becomes “0”, the packet is discarded. However, in the case of LSR that supports Ping and Trace Route, the packet in which TTL becomes “0” is not simply discarded, but the payload is checked, and when the packet is determined as the Ping or Trace Route packet, the loopback process of the packet is carried out. As described above, in the OAM of the conventional MPLS, the loopback process is realized by using TTL of the first MPLS label for transfer, and the similar method is utilized also in the OAM of MPLS-TP which is now under standardization.
On the other hand, as OAM of T-MPLS (Transport MPLS), the standardization of which has been developing in ITU-T, G.8114 (Non-Patent Document 2) is known (standardization of MPLS-TP is invalidated after the start in IETF). In G.8114, the loopback point is designated by TTL of MPLS label used for OAM identification. The OAM identification label is added inside the transfer label (second and subsequent labels) and has a format similar to that of the MPLS label for transfer, but the LSP ID which shows an ID of normal path is designated fixedly to a reserved value of “14”. In G.8114, the loopback point is designated so that the TTL which is the unused field of the OAM label becomes a desired number of hops. Since the TTL of OAM identification label is subtracted by “1” at the point where OAM can be processed, the number of points which support and process the G.8114 at the intermediate node (number of MIP−1) is set as the number of hops. The Non-Patent Document 2 discloses that this MIP is provided for both Ingress and Egress.
Furthermore, in transmission device such as ATM device used for normal leased circuit and public phone service, the loopback point is designated by a unique ID (Location ID) in the network. For example, the paragraph [0014] of the Patent Document 1 discloses that the conduction (loopback) test is carried out by designating the segments 3a, 4a, and 5a in the device (Ingress and Egress can also be separated) of the ATM switch 1 by using the Location ID of the ATM cells. The paragraph [0015] of the Patent Document 1 discloses that the response means 3b, 4b, and 5b of the ATM switch 1 references the Location ID of the loopback OAM cell 6 and sends a response when the identifier shows the segment in the device thereof.